Stable electrochromic module

ABSTRACT

An electrochromic module including a first substrate and a second substrate is provided in which the first and/or the second substrate are/is electrically conductive or are/is equipped with an electrically conductive coaling. An electrochromic polymer coating is arranged on the substrate or the conductive coaling, an ion storage layer is arranged on the substrate or the conductive coating, and a polymer gel electrolyte is disposed in an electrically in-series connection between the electrochromic coating and the ion storage layer. The electrochromic polymer is a polymer of tetraarylbenzidine and (hetero)aromatic diol, which can be switched reversibly between redox states in a voltage-controlled manner. The condensation polymer is colourless in one redox state and colored in at least two redox states. The inventive modules achieve a large number of switching cycles without an appreciable decrease in the electrochromic properties, a high electrochromic contrast and a high electrochromic efficiency with effective switching kinetics.

The invention relates to an electrochromic module comprising a first substrate, a second substrate, the first and/or second substrate being electrically conductive or having been provided with, respectively, a first electrically conductive coating or with a second electrically conductive coating, a coating of an electrochromic polymer disposed on the first substrate or the first conductive coating, an ion storage layer disposed on the second substrate or the second conductive coating and an electrically series-connected electrolyte disposed between the electrochromic coating and the ion storage layer.

It is a feature of the inventive electrochromic module that it can be switched reversibly under voltage control between more than two color states and has, after a large number of switching cycles without any significant decline in the electrochromic properties, a high electrochromic contrast and a high electrochromic efficiency with rapid switching kinetics, and is additionally leak-proof.

The electrochromic polymer used is an essentially linear condensation polymer formed from a substituted tetraphenylbenzidine and a (hetero)arylenebisphenylmethanol of the general structural formula (I), (II), (III) or (IV)

where R1 and R2 are the same or different and are each an alkoxy group, a halogen atom, a cyano group or a hydrocarbyl radical having 1-10 carbon atoms, preferably an alkyl group, an allyl group or a vinyl group, and R3 is a divalent radical of an optionally substituted aromatic or heteroaromatic compound, preferably of benzene, a hydroquinone dialkyl ether, diphenyl ether, biphenyl or naphthalene.

Electrochromic modules for use as light filters, displays, dazzle-free rear view mirrors and the like are known. These involve reversible electrochemical oxidation and reduction of redox-active materials such as tungsten oxide, viologen or various polymers such as polythiophene, polyethylenedioxythiophene (PEDOT), polyaniline inter alia, which changes the color thereof. Even though the various known electrochromic systems work quite well in individual cases, there are also a number of disadvantages. The electrochromic materials such as bipyridinium compounds (viologens) can be switched between three redox forms, reversibly from the dication to the radical cation and irreversibly to the uncharged form. In this case, the pimerization of the radical cations (formation of a n complex through the n electron planes) causes an altered absorption spectrum and has an adverse effect on the color contrast and the lifetime of the EC systems.

Stabilization materials are required, such as metallocenes and metallocene derivatives (DE 102007037619A1, US 2009/0259042A1, DE 102008024260B4) and also other compounds known, for example, from EP 1288275A2 and DE 102006061987, which, by guaranteeing a reversible anodic component reaction, ensure an improved lifetime of the cathode-switching electrochromic formulation (preferably 4,4′-bipyridinium salts) with regard to long-term contrast stability. Here, however, there are likewise problems with regards to color contrast and lifetime. In long-term studies, formation of metallocenium cations becomes perceptible through formation of a yellow-brown layer at the anode. Moreover, the addition of metallocenes to an electrochromically active formulation leads to separation processes which have been uncontrolled to date, for example the deposition of ferrocene aggregates.

Most of the electrochromic materials of significance for applications are switchable only between two colors: viologens (colorless⇄blue/violet), tungsten oxide (WO₃) (light blue⇄blue), poly-3-hexylthiophene (violet⇄blue),

polyethylenedioxythiophene (PEDOT-PSS) (light blue⇄dark blue). It is thus possible to implement only two-color filters or only monochrome displays.

Moreover, many electrochromic materials, for example WO₃ or PEDOT-PSS, are merely pseudo-colorless in thin layers, and so they are only of limited suitability for the applications where the colorless state is required within a broad wavelength range (500-1000 nm).

Numerous studies have been conducted to date with regard to organic materials using the electrochromic effect. The great advantage of the electrochromic polymers and the controllable multichromicity thereof through modification of the chemical structure, and the inexpensive production of arbitrarily thin and large-area layers both on glass or metal substrates and on flexible films and textiles.

Known polymers suitable for electrochromic applications are polythiophenes, polypyrrole, polyphenylenevinylenes and polyaniline. However, these electrochromic conductive polymers have a tendency to alterations under air, especially with regard to the electrical properties and electrochemical stability thereof, and as a result have only a short lifetime. It is therefore important to encapsulate the EC modules and to protect them from outside influences. In this context, the necessary rigid encapsulation impairs the flexibility. Moreover, such polymers have a low glass transition temperature, and polypyrrole and polyaniline, for example, have a poor solubility, and so they are processable with difficulty. These disadvantages constitute serious hindrances to the practical use thereof.

Particular polymers having di- or triarylamine units are known as hole conductors, electroluminescent materials and light-emitting materials, and also multicolored electrochromes (W. Holzer et al., Optical Materials 15, 2000, 225-235).

Examples of the use of diphenylamine and derivatives thereof as electrochromic material or in combination with anthraquinones are described in U.S. Pat. No. 4,752,119. It has been proposed that a solution of a diphenylamine and a conductive salt in a chemically stable organic solvent (preferably propylene carbonate) between two electrodes be used. A TiO₂ scattering layer was applied to an electrode, in order to better perceive the color change on the white background. As a result of the application of a voltage of 1.0 V to 1.5 V, the solution takes on a green color. If the voltage is increased to 2.2 V, a blue-green color forms in the solution. If the voltage is switched off, the system returns to the colorless ground state via diffusion. After 10⁶ switching cycles, only relatively small electrochromic deteriorations in the cell were registered. However, such systems comprising liquid media are problematic in terms of the operating temperatures and lifetime; therefore, they have to be hermetically encapsulated.

The invention according to DE 3717917 relates to a novel polymer which consists of repeat units of N,N,N′,N′-tetraphenyl-p-phenylenediamine and has electrochromic properties. The polymer is soluble in organic solvents and only becomes insoluble once it has been doped with an electron acceptor and then dedoped. This polymer film shows a yellow color in the potential region of 0.3 V (with respect to Ag/AgCl), a green color in the oxidized state of the first stage at 0.85 V, and a dark blue color in the oxidized state of the second stage at 1.2 V. An electrochromic display was produced through the following steps: a transparent glass plate was subjected to vapor deposition of an insulation film of MgF₂ (80 nm) outside the display region, then coated with the abovementioned polymer from a chloroform solution (200 nm) , subsequently doped with iodine at 100° C. and then dedoped under high vacuum. On another glass plate coated with a graphite fiber layer, a Prussian blue film (300 nm) was electrolytically deposited. Between the two glass sheets was disposed a porous background panel made from alumina, and the two electrodes were sealed. The electrolyte used was 1 mol/l solution of LiClO₄ in propylene carbonate. This electrochromic display was switched repeatedly up to 10⁵ times by applying a coloring voltage of 8 V and a lightening voltage of −8 V. In the course of this, only a small change in the amount of charge was determined in the oxidation reaction compared to the starting value. The production of the electrochromic display is a multistage operation, combined with various different technological operations (doping with iodine at 100° C., dedoping under high vacuum, electrolytic deposition of Prussian blue), which leads to increased technical complexity and investment. Furthermore, the coloring and lightening voltages of ±8 V are very high compared to conventional EC cells and are economically disadvantageous.

DE 3615379 A1 describes a dazzle-free mirror. The first electrochromic layer is formed from a conjugated polymer such as a substituted or unsubstituted triphenylamine, and the other EC layer is a transition metal oxide, such as WO₃. In the process described, a film is applied to the electrode from suitable triphenylamine monomer or polymer solutions using a coating process and is subsequently polymerized or crosslinked by means of oxidizing agents, such as iodine, antimony pentafluoride, arsenic pentafluoride or iron oxide. A further means of film formation is an electrolytic polymerization from monomer solution. For example, such a mirror consists of 4,4′-dichlorotriphenylamine polymer and WO₃ EC layers with an electrolyte solution of LiCl₀₄ in propylene carbonate with 3 % by weight of water. The reflection of the mirror in the ground state is about 70 %. In the case of application of a voltage of about 1.45 V, the mirror turns dark blue within about 4 s, and so the reflection is lowered to about 10 %. A voltage of about −0.35 V led to decoloring of the mirror. The subsequent coloring (1.1 V, 15 s) and decoloring (−0.4 V, 90 s) were stably reproducible even after 30 000 repetitions. The in situ polymerization or crosslinking of the coating film has the disadvantage that residues of the oxidizing agent in the film can lead to unwanted side reactions in the case of repeated oxidation and reduction, and as a result to an unsatisfactory lifetime of the device. Equally, it gives an additional methodological step for practical use.

Electron-rich triphenylamines have a tendency to be oxidized in the presence of oxygen and light to form an unstable radical cation, which dimerizes further to a tetraphenylbenzidine. This oxidation leads both to yellowing of the polymer layers and to a limitation in the lifetime of the EC elements. By exchange of a group in the para-phenyl position, the dimerization reaction can be significantly reduced. However, it has been published recently that the conjugated homopolymer poly(4-methoxytriphenylamine) has only a moderately stable EC effect up to about 50 cycles (G.-S. Liou et al., Journal of Polymer Science: Part A: Polymer Chemistry, (2007), V. 45, 3292-3302).

Preparation and basic electrochemical properties of polymers having aryl-substituted arylenediamine polymers are present in DE 19832943. It has been found that the electrooxidation of a solution of a 3,3′-substituted triphenyldiamine dimer polymer (TPD polymer) reversibly gives rise to a blue color.

It is desirable to use TPD and tetraarylbenzidine polymers as electrochromic materials in an electrochromic module with suitable electrolyte and a suitable ion storage layer, which ensures the performance of redox reactions with favorable cyclic periodicity and hence a stable EC effect.

It is an object of the invention to provide an electrochromic module which is perfectly colorless within a broad wavelength range ( 500-1100 nm) and, in contrast to the prior art, can be produced in few technologically simple, environmentally friendly and inexpensive steps. In addition, it is an object of the invention to display more than two color states with only one electrochromic material and, at the same time, to achieve a large number of switching cycles without any significant decline in the electrochromic properties, a high electrochromic contrast and a high electrochromic efficiency of the modules with effective switching kinetics.

The object is achieved by an electrochromic module comprising a first substrate, a second substrate, the first and/or second substrate being electrically conductive or having been provided with, respectively, a first electrically conductive coating or with a second electrically conductive coating, a coating of an electrochromic polymer disposed on the first substrate or the first conductive coating, an ion storage layer disposed on the second substrate or the second conductive coating and an electrically series-connected electrolyte disposed between the electrochromic coating and the ion storage layer, characterized in that the electrochromic polymer is an essentially linear condensation polymer which has been formed from a tetraarylbenzidine and a (hetero)aromatic diol and can be switched reversibly under voltage control between more than two redox states, the condensation polymer being colorless in one redox state and colored in at least two redox states, and in that the electrolyte (7) is a polymeric gel electrolyte.

In the context of the present invention, the first substrate, which is optionally equipped with a first conductive coating, and the electrochromic coating disposed on the first substrate or on the first conductive coating are also referred to collectively as the working electrode. Analogously to this, the second substrate, which is optionally equipped with a second conductive coating, and the ion storage layer disposed on the second substrate or on the second conductive coating are also referred to collectively as the counterelectrode.

The invention proceeds from the known structure of electrochromic modules, and the electrochromic properties of the inventive polymers in combination with a polymeric gel electrolyte and an ion storage layer are described.

The polymer, which is redox-stable in accordance with the invention, is a tetraarylbenzidine-diol condensation polymer, preferably a copolymer of the following general structural formula I, II, III or IV:

where R1 and R2 are the same or different and are each an alkoxy group, a halogen atom, a cyano group or a hydrocarbyl radical having 1-10 carbon atoms, preferably an alkyl group, an allyl group or vinyl group, and R3 is various aromatic radicals. R3therein is a derivative of aromatic or heteroaromatic compounds, preferably of benzene, hydroquinone dialkyl ethers, diphenyl ether, biphenyl, naphthalene and other aromatics and heteroaromatics, and compounds thereof.

These polymer films are perfectly colorless and (in the visible light region) transparent in the uncharged state and can be colored and decolored again by a relatively low voltage.

The fact that the polymers used in this application (in one redox state) are colorless is a great advantage over known EC polymers (polythiophenes , polypyrroles, polyphenylenevinylenes, polyaniline and PEDOT-PSS), which are colored both in the oxidized and reduced states. For many applications in which transparency of the device is required/desired outside the colored states (including displays, glazing, spectacles), they are therefore perfectly suited.

The inventive polymers have essentially a linear structure and a high glass transition temperature (T_(g)>200° C.). It is advantageous that the polymers are stable under air in the form of thin films and do not require any inert conditions in the course of processing. Moreover, they have good solubility in solvents such as dioxane, chloroform, dichloromethane, chlorobenzene and toluene, as a result of which it is possible to produce arbitrarily thin layers from solutions on glass or metal substrates, or else on flexible films and textiles, by means of spin-coaters, doctor blade technology, roll-to-roll and printing processes, and spraying methods, the layer thicknesses being between 50 nm and 1 μm, preferably 200 to 500 nm. In contrast to the prior art, the inventive polymer layers do not require any after treatment (crosslinking, polymerizing, doping, dedoping), as a result of which the technological procedure is significantly simplified. At the same time, however, they are insoluble in water, alcohols, aliphatic hydrocarbons, propylene carbonate, ionic liquids, for example ethylmethylimidazolium bis(trifluoromethylsulfonyl)imide (EMITf₂N) and in 1-ethyl-3-methylimidazolium tetrafluoroborate, which can be used as electrolyte in electrochromic modules.

According to the invention, the EC polymers form homogeneous thin layers (approx. 200-500 nm) from solutions having a polymer content of 0.5 to 30 percent by mass, preferably of 1 to 3 percent by mass, on different glasses and flexible substrates, which attain an efficiency in the EC module of up to 950 cm²/C and an optical contrast up to 55% for the blue color.

The inventive polymers are oxidized into an electrochromic module in combination with a polymeric gel electrolyte (for example based on EMITf₂N) and an ion storage layer on application of a voltage of about 0.4 V at the working electrode, as a result of which the module takes on a homogeneous orange color. Moreover, the further oxidation of the EC polymer on application of a voltage of 0.9 V in the EC module gives rise to a homogeneous blue color. In this case, the orange color state is passed through. When the voltage decreases down to −1.0 V, the EC module returns to the colorless state. More particularly, the invention provides EC modules comprising abovementioned polymers, which can be switched as desired between colorless-clear and colored (e.g. colorless/orange and/or colorless/blue) and between two colors (e.g. orange/blue), or else between three states (colorless/orange/blue).

Because of the optimal combination of EC polymer used, polymeric gel electrolyte and the ion storage layer, the inventive electrochromic modules exhibit very good switching kinetics from blue to colorless in 2 sec (optical contrast 88%) and from colorless to blue in 7 sec (optical contrast 90%, area 3.5 cm²).

In addition, the EC polymers, on application of a voltage of about 0.4 V, have a broad long-wave absorption maximum (λ=1300 nm) with an optical contrast of about 14%.

Inventive EC modules comprising EC polymers in combination with a polymeric gel electrolyte based on EMITf₂N and an ion storage layer exhibit a stable EC effect above at least 10 000 and preferably at least 20 000 colorless/blue switching cycles.

In the invention, a gel-type or polymer-based electrolyte comprising a dissolved lithium salt is used. Gel-forming polymers are, for example, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polyacrylonitrile (PAN) or poly(methyl methacrylate) (PMMA). Preferred solvents are ionic liquids such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITf₂N) . Other nonexclusive examples of solvents are propylene carbonate, mixtures of propylene carbonate/ethylene carbonate/diethyl carbonate and other carbonates. In addition, the polymer-based electrolyte contains, in a concentration of 0.1 mol/l to 1 mol/l, a lithium salt such as LiTf₂N, LiTfO (lithium trifluoromethanesulfonate) or LiClO₄ (lithium perchlorate). The conductive salt and gel-forming polymer are fully dissolved in the electrolyte and thus do not cause any coloring of the electrolyte. As well as a high conductivity (up to 6 mS/cm (EMITf₂N, LiTf₂N, PVDF-HFP)), the gel electrolyte particularly has good optical transparency in the visual range.

In a preferred embodiment of the invention, the counter-electrode comprises an ion storage layer which to an extent of more than 50% by weight, preferably more than 60% by weight, of a material selected from the group consisting of tungsten oxide, nickel oxide, cerium oxide, titanium oxide, molybdenum oxide, vanadium oxide (WO₃, NiO, CeO₂, Ti₂, MoO₃, V₂O₅) and mixtures thereof. Ion storage layers based on CeO₂—TiO₂mixed oxides, which are based on the principle of Li⁺ insertion into CeO₂, are particularly preferred. Their main task is to rapidly and fully compensate for the charge shifted to the working electrode. A conventional CeO₂—TiO₂ mixed oxide electrode having a charge storage density up to 26 mC/cm₂ using a sol-gel process was used, this already having been described in the literature [C. O. Avellane-da et al., Thin Solid Films 471, (2005 ) 100-104 , A. Verma et al. , Thin Solid Films 516, (2008 ) 4925-4933].

The invention is illustrated in detail hereinafter by figures. The figures show:

FIGS. 1-2 each an electrochromic module with multilayer structure;

FIG. 3 the electrical circuit diagram of an electrochromic module;

FIG. 4 a cyclic voltammogram of an electrochromic module;

FIG. 5 the spectral transmission of an electrochromic module in three different switching states;

FIGS. 6-7 the plot of electrical current against time in an electrochromic module during switching operations; and

FIG. 8 the time-dependent transmission during a switching operation.

FIG. 1 shows an inventive electrochromic module 10 with a first substrate 1 and a second substrate 4. The first substrate 1 and the second substrate 4 may each consist of a transparent material, such as float glass, quartz glass, a polymer film, a metal foil, or a transparent, semitransparent or nontransparent textile. The second substrate 4 may also consist of a nontransparent polymeric, ceramic, metallic or textile material. Preferably, the first and/or second substrate (1, 4) are each provided with a conductive coating, 2 and 5 respectively, formed from the same or different materials. The conductive coatings 2 and 5 consist, for example, of a transparent conductive oxide (TCO) such as tin oxide or zinc oxide, aluminum-doped tin oxide (ZAO), indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), a metal, for example gold, platinum or stainless steel, or a conductive polymer, for example poly-3,4-ethylenedioxythiophene poly(styrenesulfonate) (PEDOT-PSS). In alternative embodiments of the invention, the first substrate 1 and/or the second substrate 4 consists of an intrinsically conductive textile comprising metallic or metalized filaments. In the case of use of a substrate 1 and/or 4 made from an intrinsically conductive textile, there is no conductive coating 2 or 5.

On the substrate 1 optionally equipped with the conductive coating 2 is disposed a coating 3 consisting of the above-described electrochromic polymer, i.e. of an essentially linear condensation polymer formed from a tetraarylbenzidine and a (hetero)aromatic diol. The coating 3 is produced by applying a solution of the electrochromic polymer by means of known methods, such as spraying, doctor blade coating or spin-coating, to the substrate 1 or to the conductive coating 2.

On the substrate 4 optionally equipped with the conductive coating 5 is disposed an ion storage layer 6 consisting to an extent of more than 50% by weight, preferably more than 80% by weight, of a material selected from the group comprising cerium oxide, titanium oxide, tungsten oxide, nickel oxide, molybdenum oxide, vanadium oxide (CeO₂, TiO₂, WO₃, NiO, MoO₃, V₂O₅) and mixtures thereof, and more preferably from CeO₂—TiO₂ mixed oxide. The ion storage layer 6 is preferably produced by applying a dispersion of one of the oxides mentioned and subsequently drying and optionally sintering. Alternatively, the ion storage layer 6 is produced by deposition from the vapor phase, for example by means of CVD or PVD.

The electrochromic module 10 further comprises a polymeric gel electrolyte 7 which is disposed between the electrochromic coating 3 and the ion storage layer 6 and comprises at least one crosslinked polymer such as PVDF-HFP, PAN or PMMA, at least one ionic liquid such as 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide, propylene carbonate, mixtures of propylene carbonate/ethylene carbonate/diethyl carbonate and at least one lithium salt such as LiTf₂N, LiTfO or LiClO₄.

According to the invention, the polymeric gel electrolyte 7 is disposed between the electrochromic coating 3 and the ion storage layer 6 such that the electrochromic coating 3, the polymeric gel electrolyte 7 and the ion storage layer 6 are electrically connected in series (see fig. 3). Accordingly, also envisaged are electrochromic modules in which the substrate 1 and/or the substrate 4 consists of an intrinsically electrically conductive textile material, or one which has been equipped with a conductive coating 2 and/or 5, the polymeric gel electrolyte 7 penetrating and filling the pores of the substrate 1 and/or 4 and encasing the conductive filaments thereof.

Optionally, the electrochromic module 10 is equipped on the edge side with a seal 8. The seal 8 consists, for example, of a polymeric material and surrounds the edge of the layer of the polymeric gel electrolyte 7 . The seal preferably extends partly or fully over the edges of the substrates 1 and 4.

FIG. 2 shows a further electrochromic module 20 according to the present invention. The module 20 comprises a structured conductive coating 2A and/or a structured conductive coating 5A. In appropriate configurations, in addition, a structured electrochromic coating 3A and/or a structured conductive ion storage layer 6A are provided. The term “structured layer” or “structured coating” in the context of the invention refers to a circuitry pattern produced by means of known methods, such as photolithography. More particularly, matrix-like patterns are provided, which enable operation of the electrochromic module 20 in the manner of a display and utilization for the digitally controlled display of images and symbols.

The example which follows illustrates the production of an inventive electrochromic module.

EXAMPLE 1

A toluene solution containing 1.5 percent by mass of polymer, prepared by polycondensation of 1,4-bis(phenylhydroxymethyl)benzene and N,N′-bis(4-methylphenyl)-N,N′-diphenylbenzidine is applied to an FTO glass (i.e. glass equipped with an electrically conductive coating composed of fluorine-doped tin oxide) by means of a spin-coater, forming a homogeneous film of thickness about 500 nm (after drying). This is envisaged as the working electrode (WE) in the EC module. An ethanolic solution containing 5% by mass of water, 0.2 mol/l of ammonium cerium (IV) nitrate (NH₄)₂Ce(NO₃)₃ and 0.2 mol/l of tetraisopropyl orthotitanate (Ti(o-propyl)₄) is applied to the other FTO glass by means of a spin-coater in a thin layer, and dried at 150° C. This is repeated three times and the whole assembly is finally heated at 500° C., forming a CeO₂—TiO₂ mixed oxide. This is then used in the EC modules as the transparent counterelectrode (CE). Two coated FTO glasses (WE and CE) are then combined to give the EC module by means of a heat-sealing film. The EC module is finally filled in a glovebox with polymer electrolyte (PVDF-HFP, LiTf₂N 0.1 mol/l in EMITf₂N) at 90° C. through two small holes, and the latter are subsequently sealed.

The cyclic voltammogram of a reversible electrochemical oxidation of the EC polymer in combination with polymer electrolyte (PVDF-HFP, LiTf₂N 0.1 mol/l in EMITf₂N), demonstrated in an EC module produced according to example 1, is shown in FIG. 4 (measuring instrument: Solartron 1285, 15 mV/S). The UV-VIS transmission spectra of the same EC module in the colorless state and on application of a voltage of ±0.4 V (orange) and of ±0.9 V (blue) are shown in FIG. 5 (measuring instrument: Unicam UV 300, measured against air). Chronoamperometry measurements for the switching operations between the potentials of −1.0 V and ±0.4 V (colorless-orange) and −1.0 V and ±0.9 V (colorless-blue) are shown in FIG. 6 and FIG. 7. Here, very small current densities of 0.2-0.6 mA/cm² are observed directly after a potential switch. The switching times are obtained from time-dependent electro-optical measurements on the EC module, produced according to example 1, for colorless/blue switching operations (λ=750 nm) (see FIG. 8). Tab. 1 summarizes the electrochromic characteristics of the modules according to example 1 for orange and blue coloring.

The optical contrast is defined as the transmission difference between two (color) states at a particular wavelength. In our case, this is the transmission difference between the transparent ground state and the orange or blue states. The transmission of the EC module was measured with a Unicam UV 300 UV-VIS spectrometer against air (reference) at room temperature. The transmission is defined as the ratio of the intensity of the light beam transmitted through the electrochromic module to the intensity of the incident light beam in percent.

TABLE 1 Electrochromic properties of the module produced according to example 1. Orange Blue Characteristic (λ = 480 nm) (λ = 750 nm) Electrochromic contrast, 29% 54% Δ% T Optical density OD = log 0.274 0.61 (T_(bleaching)/T_(coloring)) Charge density, Q, (C/cm²) 2.9E−4 2.1E−3 Electrochromic efficiency 945 290 (280*, (cm²/C), η = log OD/Q 300**) *after 4200 colorless/blue switching operations **after 10 000 colorless/blue switching operations 

1. An electrochromic module comprising a first substrate, a second substrate, the first and/or second substrate being either electrically conductive or having been provided with an electrically conductive coating, a coating of an electrochromic polymer disposed on the substrate or the conductive coating, an ion storage layer disposed on the substrate or the conductive coating and an electrically series-connected electrolyte disposed between the electrochromic coating and the ion storage layer, wherein the electrochromic polymer is an essentially linear condensation polymer which has been formed, from a tetraarylbenzidine and a (hetero)aromatic diol and can be switched reversibly under voltage control between more than two redox states, the condensation polymer being colorless in one redox state and colored in at least two redox states, and the electrolyte is a polymeric gel electrolyte.
 2. The electrochromic module as claimed in claim 1, wherein the electrochromic polymer is an essentially linear condensation polymer formed from a substituted tetraphenylbenzidine and a (hetero)arylenebisphenylmethanol of the general structural formula (I), (II), (III) or (IV)

where R1 and R2 are the same or different and are each an alkoxy group, a halogen atom, a cyano group or a hydrocarbyl radical having 1-10 carbon atoms, and R3 is a divalent radical of an optionally substituted aromatic or heteroaromatic compound.
 3. The electrochromic module as claimed in claim 1, wherein the electrochromic polymer has a glass transition temperature Tg of more than 200° C.
 4. The electrochromic module as claimed in claim 1, wherein the polymeric gel electrolyte comprises at least one crosslinked polymer, at least one ionic liquid and at least one lithium salt.
 5. The electrochromic module as claimed in claim 1, wherein the ion storage layer comprises more than 50% by weight of a material selected from the group comprising tungsten oxide, nickel oxide, cerium oxide, titanium oxide, molybdenum oxide, vanadium oxide and mixtures thereof.
 6. The electrochromic module as claimed in claim 1, wherein the electrochromic polymer is switched under voltage control between three redox states.
 7. The electrochromic module as claimed in claim 1, wherein on application of a voltage in the range from 0.35 to 0.45 V said electrochromic module has a broad absorption band having an absorption maximum in the wavelength range from 1200 to 1400 nm, and an optical contrast in the absorption maximum of from 13% to 15%.
 8. The electrochromic module as claimed in claim 1, wherein the electrochromic module has a number of switching cycles with a contrast in the range from 90 to 100%, based on a starting value, of greater than 20
 000. 9. The electrochromic module as claimed in claim 1, wherein said electrochromic module has an electrochromic efficiency of greater than 600 cm²/C.
 10. The electrochromic module as claimed in claim 1, wherein said electrochromic module has an electrochromic contrast of greater than 40%.
 11. The electrochromic module as claimed in claim 1, wherein the switching time from blue to colorless is less than 2 s and the switching time from colorless to blue is less than 7 s.
 12. The electrochromic module as claimed in claim 1, wherein the electrochromic polymer forms a homogeneous layer having a thickness of 5 to 500 nm.
 13. The electrochromic module as claimed in claim 2, wherein the carbon atoms are an alkyl group, an allyl group or a vinyl group and the aromatic or heteroaramatic compound is benzene, a hydroquinone dialkyl ether, diphenyl ether, biphenyl or naphthalene.
 14. The electrochromic module as claimed in claim 4, wherein the crosslinked polymer is PVDF-HFP, PAN or PMMA, the ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, propylene carbonate, or a propylene carbonate/ethylene carbonate/diethyl carbonate mixture and the lithium salt LiTf₂N, LiTfO or LiClO₄.
 15. The electrochromic module as claimed in claim 5, wherein the ion storage layer comprises more than 80% by weight of CeO₂—TiO₂ mixed oxide.
 16. The electrochromic module as claimed in claim 6, wherein the electrochromic polymer, according to the redox state, assumes the color states of colorless, orange or blue.
 17. The electrochromic module as claimed in claim 9, wherein the electrochromic efficiency is greater than 800 cm²/C.
 18. The electrochromic module as claimed in claim 10, wherein the electrochromic contrast is greater than 60%.
 19. The electrochromic module as claimed in claim 12, wherein the homogeneous layer has a thickness of 50 to 500 nm.
 20. The electrochromic module as claimed in claims 12, wherein the homogeneous layer has a thickness of 200 to 500 nm. 